Alkali halides are attractive materials for use as windows and optical elements in IR detector and laser systems, particularly because of their high transmissivity in the far-IR (8- to 12-micro meter (.mu.m) region). These compounds, however, are fragile and are sensitive to moisture. Exposing polished surfaces of these halides to high humidity causes the optical qualities of the window to deteriorate rapidly, ultimately degrading the entire system in which they are employed. The short lifetimes of alkali halides under humid conditions require that moisture-damaged elements be regularly replaced. Aside from the inconvenience and the diminished military reliability caused by the above conditions, the need to constantly replace these elements would greatly increase the cost of these systems. Consequently, there is a need for a moisture-protective coating that would extend the lifetime of alkali halide optical components in uncontrolled environments. Although alkali halide elements may be used in enclosed systems under partially controlled environments, protective coatings are of particular importance when the systems are opened to uncontrolled (i.e., high humidity) conditions during maintenance operations.
A protective coating, when applied to elements used in certain types of far-IR detector systems, must meet the following conditions:
(1) Low absorption (less than 5%) in the 8- to 12 .mu.m region PA1 (2) Low water permeability PA1 (3) Insolubility in water and other solvents PA1 (4) Hydrophobicity, low surface energy, low wettability PA1 (5) Good adhesion and mechanical strength PA1 (6) Temperature stability over a wide range of temperatures
Previous attempts to prepare moisture-protective coatings for alkali halide windows have met with only partial success. Young (P. A. Young, Thin Solid Films 6, 423 (1970) showed that vacuum-evaporated As.sub.2 S.sub.3, which was deposited as a vitreous film, protected NaCl for 7.5 hr at 100% relative humidity (RH). The degree of protection increased as the thickness increased. Films of BaF.sub.2 and MgF.sub.2 did not prevent damage because the growth occurred through the coalescing of crystallites, which promoted porosity through voids. Damage to the surface always occurred at scratches on the underlying surface caused by mechanical polishing. Similar results were obtained by Hopkins et al. (R. H. Hopkins, R. A. Hoffman, and W. E. Kramer, Appl. Opt. 14, 2631 (1975)) who thermally evaporated CaF.sub.2 on mechanically polished NaCl. This afforded protection for 24 hr at 95% RH (27.degree. to 50.degree. C.), after which the film failed by localized moisture penetration along fine cracks in the film.
Organic polymers would appear to be excellent candidates for moisture-protective coatings for halide infrared windows. However, there are problems to be overcome in order to realize the full potential of these materials. For example, several polymers are known to be hydrophobic, but many contain functional groups that absorb in the far-IR. Linear hydrocarbon polymers, such as polyethylene, have second order absorption in the far-IR that precludes their being used. Also, thin polymer films are known to be porous. Hopkins et al, (supra) sputter deposited both polytetrafluroethylene and fluorinated polyethylenepropylene onto NaCl. These films protected the window from moisture damage in 95% RH for .about.72 hr before moisture permeated the film, dissolving the underlying surface.
Many of the problems discussed above can be minimized or alleviated by depositing the polymer in a glow discharge. This process is also called plasma polymerization (M. Millard, in Techniques and Applications of Plasma Chemistry, ed. J. R. Hollahan and A. T. Bell, John Wiley and Sons, New York, N. Y., 1974, Chapter 5). The first example of the utility of plasma-polymerized films as moisture barriers for alkali halides was reported by Hollahan, Wydeven, and Johnson. (J. R. Hollahan, T. Wydeven, and C. C. Johnson, Appl. Opt. 13, 1844 (1974). Films prepared from the monomers chlorotrifluoroethylene and tetrafluoroethylene (TFE) were deposited on CsI and NaCl, respectively, in a bell jar glow discharge reactor. Plasma-polymerized TFE protected NaCl from damage by 88.8% RH for 117 hr, at which time the testing was arbitrarily stopped. These films cannot be used in the far-IR since the C-F bonds absorb strongly at .about.8 .mu.m.
The closest prior art, known by applicant, disclosed by of Tibbitt, Bell and Shen (J. M. Tibbitt, A. T. Bell, and M. Shen, Proc. Fifth Conference on Infrared Laser Window Materials, ed. by C. R. Andrews and C. L. Strecker, U. S. Air Force Materials Laboratory Special Report, Wright-Patterson AFB, Ohio (1976), p. 206) where it is reported that plasma-polymerized ethane (PPE) showed .about.0.1 as much absorptance in the 8- to 12-.mu.m region as did polyethylene prepared by free-radical polymerization. The PPE film showed none of the absorption bands characteristic of carbon-carbon double bonds, and there was no change in the IR spectrum after a coated NaCl window was allowed to stand in air for 30 days. Dielectric loss factor measurements of the PPE coated NaCl window suggested that there was a very low uptake of water into the polymer matrix when the film was exposed to high humidity. The PPE polymer was found to be insoluble in organic solvents, stable in acid and base, and did not degrade when heated to 300.degree. C. (Polyethylene melts at 115.degree. to 135.degree. C.) (F. W. Billmeyer, Jr., Textbook of Polymer Science, Interscience, New York, N.Y., 1962, Chapter 13) depending on the density). However, it has been the experience of Applicant herein that alkali halide crystals coated with PPE film, in a manner similar to that taught by Shen et al (supra), developed moisture bubbles below the surface of the polymer coatings when exposed to high humidity environments for a prolonged period of time.